In both the National Television System Committee (NSTC) and Phase Alternating Line (PAL) standard video systems, video data may be transmitted as a single composite signal carrying both brightness and color information. Specifically, a composite video signal includes synchronization information, a luminance or “luma” (Y) component and a chrominance or “chroma” (C) component. The C component is generated by modulating U and V color components with a color subcarrier of a given color subcarrier frequency, in which the U component is the difference between the Y component and the blue color content and the V component is difference between the Y component and the red color content.
In both the NTSC and PAL systems, frames of composite video data are transmitted as two interleaved fields of lines of display pixels. Generally, each line of pixels is transmitted as a horizontal synchronization signal, a color burst signal, and the active composite video Y and C components. The horizontal synchronization signal indicates the start of the transmission of the line and gates the following color burst signal into the color synchronization circuitry. As discussed below, the color burst signal controls the recovery of the U and V color components from the received C component during demodulation. Generally, the color burst is a sample of video data represented by the subcarrier signal, which has been modulated such that the V component has a zero (0) value and the U component has a non-zero value.
A typical composite video decoder in the receiving system includes a Y/C separator stage that separates the Y and C components prior to extraction of the U and V components. For example, in an NTSC system, the C components for corresponding pixels of alternating display lines are one hundred and eighty (180) degrees out of phase. Hence, in three—line adaptive comb filtering, the pixel data from the previous and following display lines are subtracted from the pixel data from the current display line to cancel out the luma components and extract sets of C components. Similarly, the pixel data from the previous and following display lines are added to the pixel data from the current line to cancel out the C components and extract sets of Y components. Correlation between the pixel data of the current and previous display lines and between the current and the following display lines is utilized to select a set of extracted Y and C components for further processing.
A typical video decoder also includes a phase locked loop, which generates cosine and sine signals at the color subcarrier frequency for demodulating the C component of the composite video signal. Specifically, the cosine and sine signals are locked in phase and frequency to the color subcarrier frequency of color burst signal of each received line. The cosine and sine signals are locked in frequency and ninety degrees (90°) out-of-phase with respects to each other. Generally, the cosine signal demodulates the C component of the composite video signal to recover the V color component and the sine signal demodulates the C component of the composite video signal to recover the U color component, following separation of the Y and C components.
One significant problem with current Y/C separation techniques results from non-ideal phase differences between pixel data of adjacent display lines, due to factors such as clock jitter and noise. For example, in an NTSC display system, the phase difference between the C components of pixel values of adjacent display lines may not be exactly one hundred and eighty (180) degrees. Consequently, errors occur when pixel values from the adjacent are added and subtracted during the separation of the Y and C components, which can result in anomalies on the display screen.
Hence, in order to minimize anomalies in the video display systems operating on composite video, new techniques are required for addressing the problem of clock signal phase relationship during Y/C separation. These techniques should be applicable to both NTSC and PAL standard systems, although not necessarily limited thereto.